In injection moulding apparatus utilizing a stack mould design, a melt transfer system is required which transfers melt across mould levels yet which is separable to enable mould separation. The component of the melt transfer system which effects separation is referred to as a “cross over nozzle”.
In order to be effective, a cross over nozzle is provided with some means for blocking melt flow upon separation. Prior art systems include a valve gated design such as described in U.S. Pat. No. 4,212,626, a hot probe design such as described in U.S. Pat. No. 4,891,001 and a valveless melt transfer system such as described in U.S. Pat. No. 5,458,843. Each such system has particular benefits for certain types of application. Each however typically drools or leaks in one way or another.
The valve gated design utilizes a pair of nozzles which are pressed up one against the other when the mould is closed with respective nozzle orifices in registry. Each nozzle orifice has a pin which can be advanced to block its respective orifice or retracted to unblock the orifice and permit melt flow. A disadvantage with this arrangement is that it is mechanically complex. A positive driving force is required for the pin, which can be mechanical, pneumatic or hydraulic. The driving mechanisms typically require a considerable amount of space and accordingly such an arrangement may not be useable in some applications due to space constraints. There is also typically some stringing at the gate with such an arrangement. As the two pins open and close in a hot resin environment, hot resin may be trapped between the two pins causing a string to form when the mould is opened.
The valveless melt transfer design includes an expansive chamber which captures melt during mould opening. This is an effective system which requires minimal shut height yet still causes some angel hair stringing.
The hot probe design basically utilizes a heated nozzle tip to selectively allow the resin to solidify and block the nozzle or melt to free the nozzle. As it lacks a valve pin it has a tendency to drool heavily yet has the advantage of being compact and accordingly suited to an arrangement where space is limited. The hot probe design has better control at its “front” where its cooling is better controllable. The hot probe and valveless designs are referred to as being “sprue gated” as opposed to “valve gated” in view of having a sprue but no valve.
It is an object of the present invention to provide a sprue gated design without the disadvantages of being prone to heavy drooling.